In conventional LNG plants, heat transfer for cooling a natural gas feed stream sufficiently to form a liquid is effected in a heat exchanger. Natural gas can contain a wide range of compositions of species which are capable of forming solids during the cryogenic process of liquefying natural gas. Such species are referred to throughout this specification as “freezable species” and the solids formed of the freezable species are referred to as “freezable solids”.
Freezable species which are not removed prior to entering the cryogenic LNG cooling vessel precipitate and accumulate on the cold surfaces of the heat exchangers and other equipment, eventually rendering these items inoperable. When fouling has reached a sufficient level, the cooling vessel must be taken off-line for the fouling to be removed. In the process the cooling vessel, baffles or pipework can be damaged which only encourages further fouling in the next production cycle. Moreover, solids condensing on metal surfaces form an insulating film reducing thermal efficiency of the heat exchanger.
In a conventional LNG facility, pre-treatment of the natural gas is required to remove the freezable species prior to the natural gas feed stream being directed to the cooling stages to cause liquefaction. In a typical natural gas, the CO2 composition can range between 0.5% to 30% and can be as high as 70% in commercially viable reservoirs like Natuna. In a conventional LNG facility, the level of CO2 present in the natural gas is typically reduced down to the level of 50 to 125 ppm prior to the natural gas feed stream being directed to liquefaction. Another of the freezable species, namely hydrogen sulphide (H2S), is normally removed down to a level of 3.5 mg/Nm3 prior to the natural gas feed stream being allowed to enter the liquefaction stage. One of the methods typically used to remove the freezable species from the natural gas feed stream is a chemical reaction using reversible absorption processes such as absorption with an amine solvent.
This is an expensive and complex process and commonly encounters operational problems such as foaming, corrosion, blocked filters, amine degradation, and losses of amine, water and hydrocarbons. The process also consumes energy to regenerate and pump the solvent Treated gas from the amine system will be water saturated and needs to be dried to less than 1 ppm prior to liquefaction. This is normally achieved by using fixed-bed solid adsorbents such as molecular sieves.
The natural gas feed stream is sometimes pre-treated to partially remove water along with some heavy hydrocarbons by means of a pre-cooling cycle from the main refrigeration unit. Alternatively, Joule-Thomson cooling can be used if excess feed gas pressure is available. Care must however be taken to keep the gas above the hydrate formation temperature. This is again a relatively expensive process. Large insulated pressure vessels are required along with a regeneration system. Regeneration of the molecular sieve is required and this consumes energy to heat the gas. The regenerated gas must be heated prior to entering the “wet” adsorption unit, then cooled to remove water before it is recycled (usually compressed) to the inlet of the duty adsorption unit. If a molecular sieve is used to remove CO2, the regeneration gas must be disposed of or used as fuel gas.
Heavy hydrocarbons (typically C6+) are typically partially removed along with water as explained above. Where further removal is required, a cryogenic distillation column is required, with cooling provided from the main refrigerant cycle. Again, this can be an expensive and complex process, especially if the removed components are required for refrigerant make-up in a mixed refrigerant cycle.
An attempt has been made to develop a process for removing the freezable species during the liquefaction stage as described in WO 99/01706 (Cole et al. The distallative separation process of Cole et al includes a controlled freezing zone in which the freezable species both solidify and subsequently melt prior to distallative separation in the bottom half of the column. The freezable species are removed in the form of a liquid via a bottoms product enriched in the freezable species.
There are no known techniques for removing the freezable species during liquefaction with the freezable species remaining in solid form.